Process for making laminated magnetic cores



March 28, 1961 J. LEMAlRE 2,976,605

PROCESS FOR MAKING LAMINATED MAGNETIC CORES Filed NOV. 14, 1957 2 Sheets-Sheet 1 \jgigq INVENTOR Jacques Lematre BY P (1W2 PM ATTORNEYS March 28, 1961 J. LEMAIRE 2,976,605

PROCESS FOR MAKING LAMINATED MAGNETIC CORES Filed Nov. 14, 1957 2 Sheets-Sheet 2 l 46% I I W T 5g- INVENTOR c/acques L emu-ire ATTORNEYS PROCESS FOR MAKING LAMINATED MAGNETIC CORES Jacques Lemaire, Le Havre, France, assignor to Aktiengesellschaft Brown, Boveri & Cie, Baden, Switzerland, a joint-stock company Filed Nov. 14, 1957, Ser. No. 696,489

Claims priority, application France Nov. 14, 1956 11 Claims. (Cl. 29--155.61)

This invention relates to magnetic cores for electrical induction apparatus such as transformers, choke coils and the like and more particularly to an improved process for making a laminated magnetic core of cylindrical configuration.

It is well known that it is advantageous to make the core parts for magnetic circuits, such as the cores of transformers and choke coils in the form of circular cylinders. The filling of the cross sectional area formed by the circumscribed course with iron yields the best conditions if a minimum cost is a principal objective. Moreover, it is possible to obtain a sufficiently rugged construction without having to perforate the metallic lamination sheets, which would otherwise lead to unfavorable properties in the magnetic circuit.

Various arrangements have been suggested in order to obtain the above-mentioned advantages. For example, it has been proposed to make up the cylindrical core from parallel lamination sheets with their width decreasing toward the outside in such manner that the edges of the sheets lie possibly on a circumscribed circle. Also proposed has been an assembly of radially arranged sheets of varying width, increasing in number toward the outside. Finally it has been proposed to utilize an assembly of lamination sheets of equal width which have a curvature corresponding to the involute of a circle. The last mentioned construction has, to applicants knowledge, been limited to transformers with a very small output. It has not been used industrially on a larger scale for large sized transformers because it has been impossible to overcome certain difliculties. Actually, the shaping of the sheets, which are at first plane, along an involute of a circle requires considerable forces which increase as the dimension of the sheets increase for, obtaining a higher output. The internal mechanical stresses and the clasticity of the sheets present great diificulties with respect to the form-stability of the parts of the magnetic circuit formed from these sheets, which are greater the greater the dimensions of these parts become. It is then necessary to permanently install large clamps for holding the sheets together, whose dimensions hinder the production. Moreover, the original magnetic characteristics of the sheets are adversely affected by these clamping forces.

The present invention also relates to cylindrical magnetic cores formed from lamination sheets having the configuration of a partial involute of a circle and in particular to an improved process for forming such a core and wherein the core offers good conditions for cooling by establishing axially extending cooling channels for circulating a fluid coolant.

More particularly, the improved process is performed in the following manner.

(a) The plane sheets are cut into strips of equal width and length, as determined by the desired properties of the magnetic circuit and are then pre-shaped to a configuration corresponding approximately to a portion of an involute of a circle.

(b) The preformed sheets are then assembled about a United States Patent cylindrical mandrel in such manner as to form a hollow cylinder and are then pressed together by clamps until the outer diameter of the cylinder corresponds to the desired dimension, the individual sheets being forced to practically assume the form of the involute.

(c) The entire cylindrical assembly of sheets is then subjected to a heat treatment by annealing, generally at a temperature above 700 C.

((1) After the heat treatment, the clamps are removed.

The annealing process according to the invention, mostly at temperatures above 700 C. eliminates the elastic mechanical stresses and restores at the same time the magnetic properties which had been temporarily lost by the pre-shaping of the initially plane sheets into the involute configuration. The principal advantage of the heat treatment of the pro-shaped and compressed sheet assembly is that it results in great strength and rigidity which makes it possible to remove the clamps after the heat treatment without further deformation of the sheets. To prevent the necessary insulation of the sheets from being impaired by the heat treatment, the sheets are preferably provided with an insulating coating which is not damaged by the temperature used for the annealing. The heat treatment of the core parts, applied after the mechanical shaping of the sheets is of particular advantage when the sheets have an oriented crystal structure. There is no further treatment after the annealing that could again disturb the magnetic properties of the sheets which, as known, are very sensitive to mechanical de-- formation.

In certain cases, namely, when the magneticcircuit is very large, it may be necessary to increase its strength.. According to the invention, welded seems can be provided at one or both ends of the cylindrical assembly at the inner edges of the core sheets in the central opening. Without departing from the spirit of the invention, it' is also possible to provide such welded seams on the edges of the sheets at the end faces of the core, or to" cement the sheet with a temperature-resistant adhesive, or to bind them together with insulating bands.

vention.

axially extending cooling channels through the cylindrical assembly. The radial symmetry leads to a uniform cooling of each sheet over two axially extending sheet strips.

In the case where the magnetic circuit has very large dimensions it is possible, in accordance with the invention, to constitute the magnetic core from bent sheets which consist of two involute sections extending in the same, or in the opposite, direction. It is also possible for the sheets to include, between the two in'volute sections, a short radially extending section. Also, it is possible to constitute the core from a plurality of telescoped sections, each of which is produced in accordance with the invention.

The invention will become more apparent in the following specification and the accompanying drawings. As described and illustrated, the invention relates to the production of a transformer core but it will be understood that the invention is not limited to production of transformer cores but can be applied also to the production of transformer yokes or parts of a choke coil or similar electrical inductive components.

In the drawings:

Fig. l is a schematic representation, in transverse cross section of a cylindrical transformer core made in acccrdance with the invention;

This mode-- of securing has naturally no relation to the above-described application of clamps in accordance with the in-- Fig. 2 is a view similar to Fig. 1 showing a suitable construction for a comparatively large core consisting of inner and outer telescoped cylindrical assemblies, each such assembly being made in accordance with the invention and the involute curves of the sheets of both assemblies extending in the same direction;

Fig. 3 is a view similar to Fig. *2 but wherein the involute curves of the sheets of the inner and outer assemblies extend in opposite directions; I

Fig. 4 is a view similar to Fig. 2 showing another construction for larger cores but wherein the core sheets consist of two joined-involute sections and with the curvatures of the sections extending in the same direction;

Fig. is a view similar to Fig. 4 and wherein the involute sections of each sheet while extending in the same direction do not directly abut but rather are joined together by a short radially extending section;

Fig. 6 is a view of still another embodiment wherein certain of the involute sheets which form the generally cylindrical assembly extend beyond the outer periphery of the assembly so as to establish external fins for cooling purposes and wherein certain of the sheets also extend inwardly into the central opening so as to likewise provide internal fins forcooling; and

, Figs. 7 and 8 are central, longitudinal sections through cores made in accordance with the invention and which also show the associated yoke components and wherein the cores are strengthened by means of welded seams.

With reference now to Fig. l, the laminated magnetic core is seen to consist of a cylindrical assembly of ferrous sheets 1 which are pre-bent into preferably the initial sections of involute curves evolved from a circle represented by the circumference of the inner circular opening 2. However, the sheets can terminate on a circle of larger radius. The sheets are assembled into a cylindrical assembly around a cylindrical mandrel represented by the circle 2 so that they are in contact with each other. The assembly can be solid or, if desired, certain groups of sheets may be omitted thus establishing axially extending cooling channels 3 of constant width through which a fluid coolant can be circulated in surface contact with the sheets. The spacing between the sheets establishingthe channels may be maintained by insulating inserts 3a. These channels also reduce the possibility of short-circuits between the sheets over their edges from which burrs have frequently not been removed. Instead of omitting groups of sheets, an insulating layer can be provided between certain of the sheets. I After the sheets have been assembled about the mandrel into generally cylindrical form, one or more strong circular clamps 4 are applied around the assembly and radially inward pressure is applied by these clamps until the outside diameter of the assembly corresponds to the desired dimension. assembly is then placed in a furnace and subjected to an annealing treatment at a temperature generally above 700 C. This annealing process'serves to eliminate the elastic mechanical stresses caused by the clamping action and also restores the magnetic properties of the ferrous sheets which had been temporarily lost when the sheets were pre-shaped to their involute curvature.

After the annealing process has been completed the clamps 4 are removed and are no longer necessary since the cylindrical assembly will now have great strength and rigidity and the sheets will retain their shape as determined by the pressure applied by the clamps 4.

In the embodiment illustrated in Fig.2 it is assumed that the magnetic core to be desired has a relatively large diameter and is at least so large that shets of the maximum width available will not establish the desired diameter for the cylindrical assembly. In such case, the core is made up from an inner cylindrical assembly of preb ent sheets 5 of involute curvature and an outer cylindrical assembly of pre-bent sheets 6 of involute curvature.

With the clamps still in place, the

The inner and outer cylindrical assemblies are made up individually and each is processed in accordance with the basic process steps detailed with respect to the embodiment of Fig. 1. After the inner and outer assemblies have thus been completed they are assembled into their telescoped form. It will be noticed that the involute curves of the inner and outer sheets 5, 6 extend in the same direction. The inner and outer assemblies may be in contact with each other if desired, or a circumferential radial gap 7 may be left between them to provide a channel through which a fluid coolant may be circulated. The sheets 5 have an involute curve derived from the circumference of inner circular opening 8 and the sheets 6 have an involute curve derived from the circumference of the outer circle 9 representing the outer periphery of the coolant channel 7.

The embodiment illustrated in Fig. 3 is generally similar to that shown in Fig. 2 and is made up in the same manner. Hence, no detailed description appears necessary. However, as distinguished from the embodiment of Fig. 2, the pre-bent involute sheets 10 of the inner assembly extend in the opposite direction from the similar sheets 11 of the outer assembly and the sheets 10 and 11 are arranged in abutting relation.

In the embodiment illustrated in Fig. 4, the sheets which make up the cylindrical assembly are pre-bent to establish two abutting sections 12 and 13. The inner sections 12 have an involute curvature derived from the inner circle 14 constituting the central opening through the core, and the outer sections 13 have an involute curvature derived from the circle 15 on which the inner sections 12 terminate, it being noted that the involutes of the two sections 12 and 13 extend in same directions. I In the embodiment illustrated in Fig. 5, which is somewhat similar to Fig. 4, the inner sections 16 have an involute curvature derived from the inner circle 17 constituting the central opening through the core, and the outer involute sections 18 are joined to the inner secitons by means of a short, radially directed interconnecting portion 19. The outer sections 18 extend in the same direction as the inner sections 16 or they can extend in opposite directions. The arrangement according to Fig. 5 enables one to obtain, between the inner and outer sections a core part with axial gaps through which a fluid coolant can be circulated. This type of cooling in accordance with the invention is most effective because the coolant comes into contact with all faces of the sheets. It can also be well adapted to the requirements because the radial length of the interconnecting part 19 can be suited exactly to the required cooling surface and only a minimum of the iron filling factor of the magnetic cross section is lost.

In the embodiment illustrated in Fig. 6, the involute sheets 20 which make up the generally cylindrical core assembly mostly terminate at what can be regarded as the outer diameter circle 21 of the assembly. However, some of the sheets are wider than the others and extend beyond the outer diameter circle 21 to establish external cooling fins '22. If desired, others of the sheets can also be made wider and these extend inwardly of the circle 23 which defines the central opening within the core to thereby establish internal fins 24 for cooling. If desired, this same cooling principle can also be applied to the embodiments shown in Figs. 1-5.

In the embodiment shown in Fig. 7, the magnetic core 25 which can be made up in accordance with any of the embodiments shown in Figs. l6 is terminated at its opposite end faces by the yokes 26 and 27. In certain situations, it may be desirable to provide supplemental means to secure all of the sheets together. In Fig. 7, this supplemental securing action is obtained by welding a circular plate 28 at its periphery into each end of the central opening 29 through the core and which thereby welds together the inner edges of all of the sheets.

The embodiment in Fig. 8 is similar to that of Fig. 7 except that the yoke parts 30, 31 do not extend across the ends of the cylindrical assembly 32 in which case a welding seam in the form of a ring 33 concentric with the core axis may be welded across one end face of the core thereby Welding the side edges of the sheets together and a similar ring 34 welded across the opposite end face.

In all of the illustrated embodiments, the strength of the core can be increased without departing from the spirit of the invention by joining the involute sheets more closely by means of an adhesive that is resistant to the annealing process or by means of an insulating envelope.

In conclusion, it will be understood that still other embodiments of the invention can be established without departing from the spirit and scope of the invention as defined in the appended claims and that the principle of the invention can be applied to the construction of all basically cylindrical core parts for the magnetic circuits utilized in various types of electrical induction equipment, typical of which are transformers, choke coils, betatrons, synchrotrons and the like.

I claim:

1. Process for making a generally cylindrical laminated magnetic core comprising the steps of pre-bending plane sheets of magnetic material to involute curvature, assembling the inner edges of said involutely curved sheets about an inner cylindrical mandrel into a hollow cylindrical form, applying a radially inward pressure to said assembly to compress the same to the desired final diameter for the core, subjecting said assembly while under said pressure to an annealing treatment to relieve mechanical stresses and restore the magnetic properties of the sheet material temporarily lost due to the bending operation, and thereafter removing said radially applied pressure.

2. Process as defined in claim 1 for making magnetic cores and which includes the further step of Welding said sheets together at the inner edges defining the central opening in the hollow cylindrical assembly.

3. Process as defined in claim 1 for making magnetic cores and which includes the further step of welding said sheets together at the edges defining the end faces of said hollow cylindrical assembly by a circular welding line extending substantially concentric with said assembly.

4. Process as defined in claim 1 formaking magnetic cores wherein said pre-bent sheets of magnetic material include two successive involutely curved sections.

5. Process as defined in claim 4 for making magnetic cores wherein said successive involutely curved sections have the same direction of curvature.

6. Process as defined in claim 4 for making magnetic cores wherein said successive involutely curved sections have the opposite direction of curvature.

7. Process as defined in claim 1 for making magnetic cores wherein said pre-bent sheets of magnetic material include two involutely curved sections and a radially extending section interconnecting said involutely curved sections.

8. Process as defined in claim 1 for making magnetic cores wherein said involute curvatures are derived from said inner circle and the inner edges of said sheets terminate perpendicularly to tangents to said inner circle.

9. Process for making magnetic cores comprising the steps of making inner and outer hollow cylindrical assemblies each as defined in claim 1, the outer diameter of said inner assembly approximating the inner diameter of said outer assembly and telescoping said assemblies together.

10. Process as defined in claim 9 for making magnetic cores wherein the involutely curved sheets of said inner and outer assemblies have the same direction of curvature.

11. Process as defined in claim 9 for making magnetic cores wherein the involutely curved sheets of said inner and outer assemblies have opposite directions of curvature.

References Cited in the file of this patent UNITED STATES PATENTS 952,105 Fisk Mar. 15, 1910 1,345,786 Kubo July 6, 1920 1,394,901 Hobart Oct. 25, 1921 1,471,263 Hobart Oct. 16, 1923 1,654,306 Paszkowski Dec. 27, 1927 1,910,227 Austin May 23, 1933 2,359,102 Foster Sept. 26, 1944 2,382,172 Putnam et al. Aug. 14, 1945 2,468,786 Sealey et al. May 3, 1949 2,534,312 Somerville Dec. 19, 1950 2,584,564 Ellis Feb. 5, 1952 2,696,593 Dole Dec. 7, 1954 

